This invention is directed toward the drilling of a well borehole, and more particularly directed toward apparatus and methods for maintaining drilling fluid circulation while attaching joints of pipe to a drill string.
Most deep well boreholes, such as oil and gas well boreholes, are drilled with rotary drilling rigs which are well known in the art. A brief description of rotary drilling will be presented as a background for understanding the objects, apparatus and methods of the present invention.
A rotary drilling apparatus comprises a drill string terminating at a lower end with ad rill bit, and terminating at the upper end with a typically square sided joint of pipe known as a kelly. The drill string is an assembly of typically thirty foot long sections or "joints" of cylindrical pipe which are threaded together. The kelly is positioned in a fitted opening of a rotary table, and the rotary is driven by a motor thereby rotating the kelly and attached drill string and drill bit. As the rotating drill bit cuts through and penetrates earth formation, the entire drill string advances into the borehole requiring additional joints of pipe to be added to the drill string to extend the borehole. Weight is applied to the drill bit in the form of drill collars to aid in the drilling operation. Rotary drilling apparatus, or "rigs", have been used to routinely drill boreholes to depths of 25,000 feet or deeper.
The action of the rotating drill bit produces pieces of formation, or "cuttings", as the bit advances within the earth formation. These cuttings are removed from the borehole by circulating drilling fluid, which is often referred to as drilling "mud". More specifically, drilling mud is pumped from a reservoir at the surface down through the drill string and out of the drill string through openings in the drill bit. The drilling mud then is forced to return to the surface of the earth through the annulus defined by the borehole wall and the outer surface of the drill string. This return flow carries cutting from the vicinity of the drill bit to the surface where they are removed prior to returning the mud to the reservoir, or "mud pit", for recirculation. The returned mud can also contain gas from formations penetrated by the drill bit. The drilling mud typically has a density of more than twice that of water. Drilling mud performs other functions in the rotary drilling operation in addition to removing bit cuttings. These functions include cooling the rotating drill bit, lubricating the bit, and providing a hydrostatic pressure head within the borehole to prevent "blow outs" of high pressure formations penetrated by the drill bit. The drilling mud is, therefore, a critical element in a rotary drilling operation and the circulation of mud at all times is critical in controlling pressure within the well and in maintaining the physical integrity of the drilled borehole.
In prior art drilling operations, the circulation of mud is terminated when additional joints of drill string are added to, or removed from, the drill string. This is because the flow conduit from the mud pump to the drill bit is interrupted when the drill string is disconnected from the kelly to add or remove threaded joints. Although the hydrostatic pressure of the mud column remains in the borehole, the additional pressure supplied by the action of the mud pump is lost when the mud pump is shut down. Reduced pressure can threaten the integrity of the borehole where the pressure drop permits sections to cave in. Furthermore, if the weight of the mud has been adjusted so that the hydrostatic pressure of the column plus the pressure supplied by the mud pump slightly "overbalances" formation pressure, cessation of pumping can result in an "under balanced" condition thereby inviting a blow out which is extremely harmful to life and property. The results of shutting down the mud pump to add or remove joints of drill pipe can also affect the mud invasion and mud cake build-up process which, in turn, can affect subsequent production, logging and even measurement-while-driving (MWD) operations.
From the discussion above, it is apparent rotary drilling apparatus and methods are needed which will allow drilling mud to be circulated during the addition of joints to the drill string as the drill bit advances in the earth, or during the removal of joints as the drill string is removed or "tripped" from the drilled borehole. In addition, apparatus and methods are needed which will allow the mud pump to circulate mud during joint addition and removal at a pressure which is essentially the same as that supplied when the drill string is rotating.